Pein's personality

As a child Nagato was a kind but sensitive boy, prone to crying, and was traumatised by the idea he had killed another man, even to save his friend Yahiko. Scared and unsure of his place in the world he desired to help those he cared about, but never knew how. Nonetheless, he became a talented and gifted student, mastering every technique Jiraiya taught him.
As Pein, his personality is very different having become a calm, serious, aloof, and detached man. Apparently believing his childhood trauma had enlightened him to the true meaning of pain and suffering, he now believes himself a deity and evolved beyond a mere human. He now seeks to show the world the meaning of suffering by using an ultimate weapon to deter future wars by means similar to 'mutually assured destruction'. He shows no moral qualms about his actions, being quite willing to kill his own teacher in cold blood and brutally execute all of Hanzō's friends and family to ensure the Hidden Rain's compliance to his takeover. He even goes so far as to proclaim himself a 'god of peace' come to guide the world to maturity through pain.
Nonetheless, he can be quite respectful, particularly to the fallen. Given Akatsuki members' frequent bickering, he often restrains them and encourages better relations between the other members, even suggesting they mourn Deidara after his passing. After his battle with Jiraiya, he holds a moment of silence for his former teacher out of respect for his power.
Pein is also very respectful and kind to his childhood freind, Konan, who has been declared his "Angel". He even goes as far as to protect her without request or hesitation from Jiraya before his battle with his former teacher. She apparently is the only member of Akatsuki, aside from Madara, that is allowed to address him by name.